Field of Invention: Fundamentals of the Compact Disc Optical Storage Technology--a Brief Summary
Optical information storage media such as CDs rely on high accuracy writing and readout of the encoded information by the optical devices (optical pick-ups) in dedicated apparatuses such as CD-ROM drives, CD and DVD recorders and players, etc. Information in such storage media can be encoded by a variety of physical and chemical means which change the structure of the information layer. The most widely used format of digital encoding is physical alteration of the data (information) layer expressed in the form of precisely defined pits and lands, waves, bubbles, etc. The accuracy of the writing and readout of the encoded information depends upon fidelity of the laser optics device for recording and playback as well as upon optical characteristics of the transparent layer of plastic (usually polycarbonate) of the CD/DVD.
While sophisticated error correction algorithms, e.g., Cross Interleave Reed Solomon code--CIRC, are used to compensate successfully for the inevitable random and spike errors during optical storage media readout, significant variables exist in the ability of CD players and CD-ROM drives to accurately reproduce all of the encoded information. Variations in the quality and light transmitting characteristic of the transparent polycarbonate layer of the optical storage media also contribute to such inaccuracies.
The optical characteristics of the transparent polycarbonate layer are critical also during the recording process on recordable and re-writeable CD-R/CD-RW/DVD for the optimization of the geometry of the physical traces of the laser burning action on the recordable information layers in such recordable media.
In some cases, variations in the optical disc manufacturing process, the efficiency of error correction algorithums used as well as in optical and mechanical properties of the readout devices introduce noticeable errors in the readout. For reasons well understood by those versed in the art description of which is beyond the coverage of this submission, low amplitude signals as well as those of high frequency and short duration are particularly prone to reproduction distortions due to the less than optimal error correction of such encoded sounds during readout (J Halliday, 1996).
Likewise, during recording on optical discs in appropriate recorders, variation in or the presence of noise in the laser beam characteristics will result in distortion of timing and the pit geometry to a relatively greater extent affecting the smaller pits rather than longer pits (J Halliday, 1996). Consequently, the information content carried in the smaller pits (more abundant in encoded high frequency audio signals) will be disproportionately affected during playback (J Halliday, 1996) . On the other hand, strong signals of high amplitude and relatively longer duration are less prone to be distorted (see for example: Pohlmann, 1995).
In the case of audio optical storage media (eg CDs), such deficiencies are perceived as frequency distortions, audible imperfections of sound resulting in apparent loss of the encoded information. Various causes related to the laser light transmission process and reflection of the beams from the data layer of the optical storage media via the transparent (polycarbonate) layer of the optical storage media may account for this deficiency.
Among such causes can be, but not limited to, the unwanted light scattering, diffraction and reduced reflection from the data layer, reflection from an interfering object, media and surfaces not intended to produce these optical aberrations.